As magenta dye image-forming couplers (hereinafter referred to as "magenta couplers"), various pyrazolone derivatives are known. However, these pyrazolone derivative couplers contained in color photosensitive materials show low coloring efficiency (low conversion rate of couplers to dyes) and so-called 4-equivalent magenta couplers having an unsubstituted coupling active position usually form dye in an amount of only about 1/2 mol per mol of the coupler.
For improving the coloring efficiency, so-called 2-equivalent magenta couplers having a substituent at the coupling active position of a pyrazolone type magenta coupler and are capable of splitting off the substituent in a color developing step are known as disclosed in, for example, U.S. Pat. Nos. 3,311,476, 3,419,391, 3,617,291, 3,926,631, etc. Also, as magenta couplers each having a substituent bonded to the coupling active position of a magenta coupler through a sulfur atom, couplers having a thiocyano group are described in U.S. Pat. No. 3,214,437; couplers having an acylthio group or a thioacylthio group are described in U.S. Pat. No. 4,032,346; couplers having an arylthio group or a heterocyclic thio group are described in U.S. Pat. Nos. 3,227,554 and 3,701,783, and in Japanese Patent Publication No. 34044/78; and couplers having an alkylthio group are described in West German Patent Application (OLS) No. 2,944,601.
However, it has now been clarified by the inventor's detailed investigations that in the magenta couplers described in the foregoing U.S. Pat. Nos. 3,227,554 and 3,701,783, the couplers having an arylthio group at the coupling active position are insufficient in light fastness in this age of rapid progressing improvement of the properties of color photosensitive materials when the couplers are used in the color photosensitive materials and color images are formed.
Also, it has been clarified that when the magenta couplers releasing an arylthio group as described in Japanese Patent Publication No. 34044/78 are used for color photosensitive materials, the color images formed are insufficient in light fastness.
Furthermore, the magenta couplers releasing an arylthio group as described in Japanese Patent Application (OPI) No. 35858/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") are excellent couplers in the point of overcoming the above-described disadvantages of the conventional couplers. However, the above-described conventional magenta couplers releasing an arylthio group have a disadvantage of reducing the coloring property when the color photographic materials containing the magenta couplers are processed by a color developer containing an alkaline earth metal salt such as a salt of calcium, magnesium, etc. This becomes a fatal demerit when the color developer is prepared using water containing a large amount of an alkaline earth metal salt, i.e., hard water. A color developer usually contains various chelating agents for covering these alkaline earth metal atoms in an amount of 1.times.10.sup.-3 to 4.times.10.sup.-3 mol/liter and the amount is sufficient for covering the alkaline earth metal atoms. However, when color photosensitive materials containing the magenta couplers having an arylthio group as a releasable group are processed by the color developer, the coloring property is reduced even by the above-described addition amount of the chelating agent, whereby a sufficient coloring property cannot be obtained.